1. Field of the Invention
The present invention relates to active pixel image sensors. More particularly, the present invention relates to full-color detector groups and arrays that use semiconductor material to chromatically filter light vertically and sense multiple wavelength bands at each cell or pixel location in an image sensor. The present invention also relates to such color detector groups configured in arrays and combined with circuitry so as to permit operation in various color subsampling modes as well as in full-color readout modes
2. The Prior Art
MOS active pixel sensors are known in the art. Multiple wavelength active pixel sensors are also known in the art. One type of multiple wavelength active pixel sensor employs red, green, and blue sensors disposed horizontally in a pattern at or near the semiconductor surface. Color overlay filters are employed to produce the color selectivity between the red, green, and blue sensors. Such sensors have the disadvantage of occupying a relatively large area per pixel as these sensors are tiled together in a plane. In addition, reconstruction of a color image from such a sensor is computationally intensive and often results in images with artifacts, defects, or inferior resolution.
Another type of multiple wavelength pixel sensor employs more than one sensor in a vertically-oriented arrangement. An example of an early multiple wavelength vertical pixel sensor for detecting visible and infra-red radiation is disclosed in U.S. Pat. No. 4,238,760 to Carr, in which a first diode in a surface n-type epitaxial region is responsive to visible light and a second buried p-region in an underlying n-type substrate is responsive to infrared radiation. Contact to the buried photodiode is made using deep diffusion processes similar to diffusion-under-film collector contact diffusion common in bipolar IC processing and for RCS reduction. The disclosed device has a size of 4 mils square. An alternate embodiment employs V-groove contacts to contact the buried p-type region of the infra-red diode.
The device disclosed in the Carr patent has several shortcomings, the most notable being its large area, rendering it unsuitable for the image sensor density requirements of modern imaging systems. The technology employed for contact formation to the buried infrared sensing diode is not suitable for modern imaging technology or extension to a 3-color sensor.
A particular example of a three-color visible-light prior art vertical pixel sensor group is disclosed in U.S. Pat. No. 5,965,875 to Merrill in which a structure is provided using a triple-well CMOS process wherein the blue, green, and red sensitive PN junctions are disposed at different depths beneath the surface of the semiconductor substrate upon which the imager is fabricated.
This prior three-color sensor group permits fabrication of a dense imaging array because the three colors are sensed over approximately the same area in the image plane. However, this structure has several shortcomings. First, this pixel sensor group uses a reverse-polarity central green-sensitive PN junction, requiring modified circuits or voltage ranges, possibly involving PMOS transistors in addition to the usual NMOS transistors, to sense and read out the green channel. This requirement disadvantageously increases sensor area and complicates support circuits in the array. The added circuit complexity makes it difficult to make an image sensor array that has the flexible color readout capabilities disclosed in this invention and makes it impossible to achieve the small pixel sizes required by many modern electronic imaging applications.
A color detection active pixel sensor is disclosed in U.S. Pat. No. 6,111,300 to Cao et al. Cao discloses a color pixel that uses a PIN photodiode above the pixel to attempt to collect blue light, and two additional semiconductor junction diodes within the semiconductor substrate which are designed to detect green and red light. Among the shortcomings of this disclosure are difficult and non-standard manufacturing techniques, use of structures that prohibit high density of pixels, no ability to select different colors for read out, and inability to perform detection of three or more colors using a monolithic semiconductor substrate.
Findlater et al. (“A CMOS Image Sensor Employing a Double Junction Photodiode,” K. M. Findlater, D. Renshaw, J. E. D. Hurwitz, R. K. Henderson, T. E. R. Bailey, S. G. Smith, M. D. Purcell, and J. M. Raynor, in 2001 IEEE Workshop on Charge-Coupled Devices and Advanced Image Sensors, IEEE Electron Devices Society (2001)) disclose an active pixel sensor that employs a double junction photodiode in conjunction with an organic filter overlay. The performance of this sensor is limited by the poor color response of the double junction photodiode and by the fact that the n-well forms the cathode of both photodiodes, making this pixel design very susceptible to non-linear crosstalk between the color channels. Additionally, the authors cite non-uniformity and process/fabrication constraints that limit the performance and potential benefits of this design.
None of the prior art discussed above provides any mechanism for dynamic control of the color filter array pattern. Also, the aforementioned prior art does not enable choosing between full-measured color and sub-sampled color. Further limitations are that the prior art discussed above does not disclose a practical way to make a color imaging array containing more than three distinct spectral sensitivities.